1. Field of the Invention
The present invention relates generally to a variable venturi carburetor for an engine in which the cross-sectional area of venturi portion automatically changes according to the amount of intake air to keep the flow rate of intake air, that is, the vacuum generated at the venturi portion at a constant level, regardless of the amount of intake air, the carburetor of this type being called a constant vacuum carburetor. Further, in the carburetor of this type, the metering jet portion of fuel also automatically changes according to the amount of intake air to keep the mixture at a constant air-to-fuel ratio at all times. The present invention relates specifically to a variable venturi carburetor of constant vacuum type in which a stable, uniform mixture can be obtained throughout the venturi portion even when the cross-sectional area of venturi is relatively small, that is, when the engine is running at a low speed or being idled.
2. Description of the Prior Art
Variable venturi carburetors or constant vacuum carburetors are well known. The variable venturi carburetor is usually attached to an intake passage on the upstream side from a throttle valve. The venturi thereof is formed between a fixed venturi portion and a movable venturi portion. The fixed venturi portion includes a nozzle body having a nozzle at one end portion thereof, the nozzle body being connected to a float chamber to supply fuel from the float chamber to the intake passage. The movable venturi portion includes a suction cylinder, a suction piston the inner space of which is partitioned into an atmospheric pressure chamber and a vacuum chamber, and a suction spring.
The suction piston serving as the movable venturi portion moves to or away from the fixed venturi portion, in dependence upon the force balance determined by pressure difference between the atmospheric pressure chamber and vacuum chamber, the urging force of the suction spring, and the weight of the suction piston, so that the cross-sectional area of the venturi changes according to the amount of intake air to keep vacuum at a constant level at the venturi portion. Further, at the center of the lower end surface of the suction piston, a tapered jet needle is fixed so as to pass through a central hole formed in the needle body. Therefore, when the suction piston moves to or away from the fixed venturi portion, the metering jet portion formed between the jet needle and the nozzle portion of the nozzle body varies to keep the mixture obtained at the venturi portion at a constant air-to-fuel ratio.
In the prior-art variable venturi carburetor as described above, however, since the venturi portion is formed by two oppositely arranged flat surfaces, that is, by the lower flat end surface of the suction piston and the upper flat surface of the fixed venturi portion, in the case where the amount of intake air is small and therefore the movable venturi portion closely approaches the fixed venturi portion, the cross section of the venturi portion becomes a slender rectangle in shape within the intake passage. Therefore, when fuel is jetted from the nozzle portion to the venturi, since fuel tends to concentrate to the middle portion of the rectangular venturi portion, fuel is not uniformly mixed with the air passing through the venturi therethroughout in particular on both the sides of the venturi remote from the nozzle portion. Therefore, there exists a problem in that the mixture is not uniform or the air-to-fuel ratio is uneven and therefore the engine is not driven stably especially when the amount of intake air is small or when the engine is running at a low speed or being idled.
A more detailed description of the prior-art variable venturi carburetor will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS.